The present invention relates to a method for measuring living tissues non-invasively by using nuclear magnetic resonance (NMR), and in particular to a method for measuring the chemical-shift-resolved spatial distribution of a predetermined nuclear species within the living tissues at high speed.
A slight shift (chemical shift) is produced in the nuclear magnetic resonance frequency of one nuclear species depending upon the chemical bond state in which the species is embraced. In the field of NMR imaging, imaging of the density destribution of protons (.sup.1 H) has mainly been conducted so far. In recent years, an imaging method in which the chemical shift information can be distinguished by combining spectroscopic techniques has been developed. Since several kinds of phosphorus compounds play an important role in the metabolic process, it is expected that the chemical shift imaging of phosphorus 31 (.sup.31 P) would significantly contribute to the biochemical diagnosis of living tissues.
A chemical shift imaging method using multidimensional Fourier imaging has been proposed by A. A. Maudsley et al "Spatially Resolved High Resolution Spectroscopy by Four-Dimensional NMR", Journal of Magnetic Resonance, Vol. 51, 1983, p.p. 147 to 152. When imaging of an x-y plane having picture elements as many as (N.sub.x .multidot.N.sub.y) is to be conducted by this method, it is necessary to conduct N.sub.x and N.sub.y phase encoding operations along the x and y axes, respectively and measure spectroscopic free induction decay (FID) signals. As a result, the imaging method needs a measurement time which is approximately hundred times that of the conventional NMR imaging since N.sub.x and N.sub.y are of the order of 100.
U.S Pat. No. 4,361,807 has disclosed a method, where a periodically inverting field gradient is superimposed on a static magnetic field to produce an echo train containing both positional dispersion information and chemical shift dispersion information and the measured echo train is Fourier-transformed. Concurrently with the direction inversion of the magnetic field gradient, a signal representing the nuclear spin information at a location on the spatial coordinate axis moves between two different positions on the frequency axis. If the measured signal is directly Fourier-transformed, therefore, a trouble is incurred in the analysis of the spatial information.